Quick questions on Ecological succession and keystone species: QCE Biology Unit 3

Primary succession begins on a substrate that has no existing soil or biological community. Examples of starting points: a fresh lava flow, glacial moraine, a newly formed coral cay, bare rock exposed by retreating ice.

Secondary succession occurs after a disturbance that removes the existing community but leaves the soil intact. Examples: a bushfire, a cyclone, a cleared paddock left to regenerate.

Pioneer species share traits that suit them to harsh, open conditions: tolerance of low nutrients, rapid life cycles, wind-dispersed seeds, high reproductive output.

A keystone species has a disproportionately large effect on its community relative to its abundance. Three common modes of keystone action:

The term keystone species comes from Robert Paine's 1966 experimental removal of the starfish Pisaster ochraceus from a rocky intertidal community in Washington State. With the starfish removed, mussels (Mytilus) outcompeted other species and reduced the community from 15 species to about 8. Pisaster was less than 1 per cent of the biomass but maintained the diversity of the entire community.

1. Pioneer community. Lichens and mosses colonise bare rock. Lichens are symbioses of fungi and algae or cyanobacteria.

Top predators suppress mid-level consumers. Example: the dingo (Canis dingo) in arid Australia. Dingoes suppress red fox and feral cat populations, indirectly protecting small mammal diversity.

A species that physically modifies the environment. Example: the bilby (Macrotis lagotis) and other digging marsupials turn over large volumes of soil each year, mixing organic matter, capturing leaf litter and creating microhabitats. Their loss across most of mainland Australia has reduced soil function in arid ecosystems.

A species whose mutualistic role supports many others. Example: flying foxes (Pteropus species) pollinate and disperse seeds for dozens of eucalypt and rainforest tree species along the east coast. Their decline reduces seed dispersal distance and forest regeneration.

The defining difference is the starting substrate. If soil exists at the start, it is secondary. If you are starting on bare rock or lava, it is primary.

Climax communities depend on climate, soil and disturbance regime. Fire-regime change can shift the climax.

Many abundant species are dominant but not keystone. A keystone species is influential beyond its abundance.

The removal of one keystone often changes a chain of species, not just the next trophic level.